Chlorine dioxide (CD or ClO2) was discovered in the early 1800's, and has been approved for a wide variety of commercial disinfecting/sterilizing applications by the EPA, FDA and USDA. Due to its demonstrated efficacy with respect to a wide variety of contaminated surfaces, ClO2 has been called the ideal biocide and the ability of chlorine dioxide to reduce or eliminate microbes, e.g., bacteria, viruses, fungi, mold spores, algae and protozoa, at relatively low concentrations is well-documented. Because ClO2 inactivates microorganisms by oxidizing critical components of a microorganism's membrane proteins, tolerance to ClO2 does not develop, making it an ideal disinfectant/sterilant for repeated-use applications such as in a hospital environment.
ClO2 is a green-yellowish gas with a chlorine-like odor; however ClO2 is a neutral chlorine compound. ClO2 is a small, volatile and very strong molecule. In diluted, watery solutions ClO2 is a free radical. At high concentrations it reacts strongly with reducing agents. Chlorine dioxide is an unstable gas that dissociates into chlorine gas and oxygen gas readily. Further, ClO2 may be photo-oxidized by sunlight and therefore decontamination applications generally proceed in the absence of light. The end-products of ClO2 neutralization/degradation reactions are chloride (Cl—), chlorite (ClO—) and chlorate (ClO3−).
ClO2 is not as reactive as ozone or chlorine and it generally reacts only with sulphuric substances, amines and some other reactive organic substances. In comparison to chlorine and ozone, less chlorine dioxide is required to obtain an active residual disinfectant. It can also be used when a large amount of organic matter is present in the environment.
A significant drawback of ClO2 is that it is explosive under pressure, thus making it difficult to transport. It cannot be transported in liquid phase or under pressure; hence it is typically manufactured on site (in situ). ClO2 is usually produced as a watery solution or gas. It is produced in acidic solutions of sodium chlorite (NaClO2), or sodium chlorate (NaClO3). Sodium chlorite, chlorine gas (Cl2), sodium hydrogen chlorite (NaHClO2) and sulphuric or hydrogen acid are typically used for the production of chlorine dioxide on site. In the presence of sunlight, ClO2 in air will decompose to chlorine and oxygen. The chlorine will react with any moisture in the air to form a hydrochloric acid mist. If the concentration of ClO2 in air in a confined space is above 10%, the chlorine dioxide is at an explosive concentration and can be ignited by almost any form of energy such as sunlight, heat or sparks, including for example, static electrical energy. Concentrations above 40% will generate a decomposition/shock wave if set off by any ignition source.
Other decontamination systems which exploit the beneficial properties of ClO2 fumigant are known in the art. However, these systems generally suffer from production of excess humidity with the fumigant, resulting in production of hydrochloric acid mist and potential to corrode electronic equipment, making the system inconvenient for large-scale building decontamination, since removal of corrosion-sensitive articles must be effectuated prior to decontamination. Further, even when corrosion-sensitive articles are removed from the area, metallic structural components of buildings may be affected. In addition to the corrosive effects of moisture, salts existing as by-products of ClO2 generation reactions and often present in the fumigant, are known to cause damage to structures and articles undergoing decontamination. This is particular problematic to areas that must be repeatedly disinfected, such as in the medical/hospital context, since the damaging effects accrue.
U.S. Pat. No. 8,524,167 (the '167 patent) discloses a ClO2 decontamination system, however it suffers from failure to provide mechanisms for removal of byproducts and relies on humidification as a necessary aspect of effective ClO2 fumigant decontamination, going so far as to add a humidifier to a decontamination chamber. The '167 patent system is unsuitable for corrosion-sensitive articles and environments. A critical consideration is that that the registered concentration of ClO2 cannot be trusted, since chlorine gas is known to influence the sensors toward detection of chlorine dioxide and to result in artificially high concentration read-outs. Chlorine gas is produced as a result of the humidification. Further, the '167 fumigant scrubber relies heavily on carbon, which is rendered less effective by the presence of water. Notably, the use of carbon filtration with non-degraded ClO2 can create an explosive potential because ClO2 can build up in the carbon pores in problematic concentrations. Hence, the use of carbon as a primary neutralizer/scrubber presents a significant fire and safety hazard.
Known ClO2 fumigant systems generally utilize a reaction sachet (bag) for generation of the gas with water, and sparging of the gas product from the liquid. The result is that acid vapor and chlorine gas are often present in the CD fumigant. As noted, both of these gases are highly corrosive to metals, and chlorine, in particular, is incompatible with many non-metallic substances as well. Neutralization of the fumigant is complicated by the presence of these additional toxic gases. Prolonged treatment time results where multiple passes are required for neutralization.
In a highly publicized recent decontamination effort by the U.S. government, a ClO2 fumigant system was employed to decontaminate a building contaminated with Anthrax spores that were released from a letter opened in a mail room. The building was tented prior to fumigation and sparged ClO2 gas was pumped into the building's heating, ventilating and air conditioning (HVAC) system to achieve a target concentration of 500 ppm at 75° F. and 75% relative humidity for 18 hours. Biological indicators (BI) comprising standard b. subtilus spore strips were placed throughout the facility. (Standard BIs contain 106 natural pathogens—sufficient to indicate a maximum 6-log spore reduction, however the BI's were not normed to Anthrax). Hence the effectiveness of decontamination was also tested via swipe sampling. Reportedly, the original plan to neutralize the ClO2 with ascorbic acid was abandoned when very high concentrations of chlorine gas were found localized throughout the building. Because the presence of chlorine molecules interferes with ClO2 monitoring to yield false high concentration readings, it was presumed therefore that concentration targets were not actually met and the procedure had to be repeated three times over 9 months for a total cost of nearly 50 million dollars to U.S. taxpayers.
Clearly there remains a need in the art for safe and effective ClO2 fumigant decontamination systems that minimize use of water, minimize agitation/degradation of the CD fumigant, and that avoid dispersal of water vapor, acid and chlorine gas along with the fumigant.